Polyvinyl halide (inclusive of vinyl halide homopolymers as well as copolymers of vinyl halide with less than about fifty percent of an ethylenically unsaturated comonomer copolymerizable with the vinyl halide) is a widely used thermoplastic having a number of favorable technological properties. However polyvinyl halide, e.g. polyvinyl chloride, especially rigid polyvinyl halide, breaks on impact very easily at low temperature (i.e. at ambient temperature or lower). It is therefore generally necessary to improve its impact resistance by addition of an appropriate polymer additive or additives (conventionally termed "polyvinyl halide impact modifiers").
The selection of an appropriate impact modifier for polyvinyl halide is highly empirical since, in addition to imparting enhanced impact strength to the polyvinyl halide substrate, the modifier should fulfill additional requirements, namely:
1. small quantities of the modifiers must suffice; (i.e. the modifier must be effective in imparting impact resistance when present in a minor proportion in the mixture of polyvinyl halide and impact modifier)
2. the good properties of the polyvinyl halide should not be adversely affected;
3. the modifiers must be incorporated into the polyvinyl halide easily;
4. the modifiers must be stable towards light and ageing if the polyvinyl halide is to be used in the open over a long period;
5. when the high impact polyvinyl halide is processed, its impact resistance must also be substantially constant when the processing conditions (e.g., temperature, time, shearing effects) vary;
6 if a transparent or translucent product is desired, light transmission (i.e. transluscency) and transparency must not be substantially impaired;
7. conventional additives and stabilizers must be usable;
8. the high impact modifiers must be physiologically acceptable.
A particularly serious defect in many polymers which might otherwise be thought to be suitable as impact modifiers for polyvinyl halide is incompatibility of the polymer additive with the polyvinyl halide. Such incompatible polymers when mixed with polyvinyl halide and subjected to elevated temperature processing conditions (as in molding) do not form stable polymer blends with the polyvinyl halide substrate, i.e. on fusion they form a polymer phase separate from the polyvinyl halide phase. The incompatible additive phase when present can impair the impact resistance as well as the light transmission and transparency of the polyvinyl halide composition. This incompatibility often manifests itself by formation, on molding of the composition, of a solid exudate on the surface of the composition which imparts an undesirable rough or lumpy handle to the composition.
Among materials which have found acceptance as polyvinyl halide impact modifiers are the polymer compositions, consisting essentially of acrylonitrile, 1,3-butadiene and styrene monomer units which are known generically as "ABS" polymers. Unfortunately these polymers, when prepared for use as polyvinyl halide impact modifiers, are relatively costly. This is especially so if a transparent or transluscent product is desired. In such instances, an ABS polymer must be prepared or synthesized under careful control so as to have about the same refractive index as the vinyl halide resin and thereby maintain the transparency or transluscency of the vinyl halide resin.
Accordingly, it would be technologically desirable to replace a portion of the ABS polymer in an ABS-modified polyvinyl halide composition by a polymer which meets the requirements for impact modification and which, desirably, is also readily available at a refractive index about the same as that of polyvinyl halide.
A readily available class of polymers, namely the block thermoplastic elastomers of a hydrocarbon alkadiene of 4 to 10 carbon atoms and a mono-alkenyl-substituted aromatic compound of the benzene or naphthalene series containing up to 20 carbons (as typified by the block polymers of 1,3-butadiene or isoprene and styrene) is known to have a refractive index about that of the polyvinyl halides (as indicated by "Modern Plastics Encyclopedia 1974-1975", Vol. 51, No. 10A, October 1974, page 563, entry 35 at the fourth and seventh vertical columns). However, as shown in the Examples set forth below, these block polymers are found in general to be incompatible with polyvinyl halide.
The following prior art is of interest but does not disclose or suggest the invention.
U.S. Pat. No. 4,048,254, (E. L. Hillier et al., issued Sept. 13, 1977) discloses polymer mixtures containing two block radial elastomers of 1,3-butadiene and styrene having different diene contents together with 5 to 75% (based on the weight of the block polymers) of certain other polymers, namely epoxide polymers, acrylic polymers, styrene-acrylonitrile polymers, polycarbonates, polyolefins, polystyrenes, polyvinyl chloride, olefin/polyvinyl chloride copolymers, preferably polyether- and polyester-urethane polymers, methyl methacrylate-styrene-1,3-butadiene copolymers, and methyl methacrylate-acrylonitrile-styrene-1,3-butadiene copolymers, as well as mixtures thereof. The compositions have a sufficient clarity, hardness, tensile strength and elongation to render them suitable for use in medicinal applications particularly as a replacement for vinyl halide polymers. Hillier et al. do not disclose an ABS polymer as a component in their polymer mixture. Moreover, the physical properties listed for the mixtures do not include impact resistance.
N. Platzer, Chemtech, October 1977, pages 634-641 discloses the use of the aforementioned block polymers as components for enhancing the impact resistance of high impact polystyrene and acrylonitrile-1,3-butadiene-styrene (ABS) copolymers. Also it is known to employ these block polymers as impact modifiers in polyethylene and polypropylene. However, the prior art does not disclose ABS polymer-modified polyvinyl halide containing the aforementioned block polymers as impact modifier components.